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And only a lack of technical know-how would stop Trump pressing the button to drop a nuclear bomb himself if he was resolute in doing so.

This is in spite of the president of the United States being unable to declare war, with the Constitution enshrining that responsibility to Congress.

It means that North Korea is wrong in declaring overnight that Donald Trump's tweets were a "declaration of war".

The North Korean foreign minister made the assertion in New York yesterday, while attending the UN General Assembly.

"All the member states and the whole world should clearly remember it was the United States that first declared war on our country," Ri Yong-ho told reporters.

"Since the United States declared war on our country, we will have every right to take counter-measures including the right to shoot down US strategic bombers even when they are not yet inside the airspace border of our country."

Ri was speaking in response to Trump's provocative tweet on Saturday.

"Just heard Foreign Minister of North Korea speak at U.N. If he echoes thoughts of Little Rocket Man, they won't be around much longer!" Trump wrote.

Trump made the tweet in between criticising NFL players for kneeling in protest during the national anthem.

"By a strictly legal point of view, what Donald Trump says does not amount to a declaration of war," Dr Gill told nine.com.au.

"That's supposed to be the purview of Congress.

"It's become a very odd, empty clause in the constitution that is no longer utilised."

Congress's formal power to declare war is, in Dr Gill's words, an anachronism.

The legislature has only declared war five times, with the last instance to enter World War II in 1941.

Since then America has been involved in Korea, Vietnam, Iraq and Afghanistan, as well as other smaller conflicts.

"Every president since Kennedy has arrogated to themselves a greater and greater ability to circumvent potential obstacles to using military force," Dr Gill said.

"Donald Trump is not the first and wouldn't be the last to wage war as we know it, without a formal declaration."

And while Trump's recent statements have been "unduly provocative", a two-letter word makes all the difference.

"One of the aspects of (Trump's) recent statements that a lot of people have missed, is he quite clearly states that if America or American allies are attacked, or if America must defend itself or its allies against an attack, then North Korea would be destroyed," Dr Gill said.

Complicating the paperwork of a war declaration is an awkward oddity of history: North Korea and the US are not technically at peace right now.

North Korea and the United Nations Command signed an armistice at the end of the Korean War, and not a peace treaty.

The armistice created the demilitarised zone, put a ceasefire into place and allowed for the exchange of prisoners-of-war.

It was made to "insure a complete cessation of hostilities and of all acts of armed force in Korea until a final peaceful settlement is achieved".

The problem is, a final peaceful settlement has never been achieved after 64 years of ceasefire.

But the distinction between a peace treaty and a ceasefire may effectively just be on paper. A war between North Korea and any other country won't be inhibited by previous agreements, but rather, the desire not to see mass bloodshed and destruction.